Microwave excited plasmas are used for many different processes, e.g. plasma processing (etching or chemical vapor deposition) of workpieces, such as silicon wafers, diamond deposition onto a workpiece, plasma treating of coatings on a workpiece, ion implantation of workpieces, and implanting particles in workpieces. A microwave field supplied to a vacuum chamber responsive to ionizable gas interacts with the ionizable gas to form a plasma for processing the workpiece.
The microwave energy supplied to the chamber is typically derived from a relatively high power source, such as a magnetron having a rating of at least 1 kW. For certain types of operation, the vacuum chamber includes an electron cyclotron resonance structure including a DC magnetic field source. The magnetron often has a frequency of 2.45 GHz and the DC magnetic field in the chamber produces a magnetic field of 875 Oersteds. The stated frequency and DC magnetic field establish the electron cyclotron resonance phenomenon which is most efficient when circularly polarized microwave energy is supplied to the chamber. For other applications, the electron cyclotron resonance phenomenon is not needed and linearly polarized microwave fields are supplied to the cavity.
The microwave excited plasma absorbs significantly different amounts of microwave energy as a function of plasma conditions, such as its pressure, gas composition and gas flow, and the electric and magnetic fields coupled to and established in the plasma. These conditions, and therefore microwave power absorption by the plasma, vary as a function of time while a workpiece is being processed by the plasma. Hence, the plasma is a highly variable load for the microwave energy coupled to the chamber and absorbed by the plasma. Precautions must be taken to counteract the variability of microwave energy absorption by the plasma. Otherwise, a plasma excited by microwave energy is likely to be quite inconsistent and variable with regard to a number of parameters (particularly plasma flux density) as a function of time and space.
Prior art microwave power delivery devices for plasma and non-plasma loads have concentrated on matching the load to a microwave source. A reactive matching network, frequently referred to as a tuning network, is usually connected between a microwave source and the load. Typically, the reactive tuning network includes a waveguide including a three or four stub tuner or a sliding short circuit element. The tuning network transforms the impedance of the load to an impedance substantially equal to the impedance of the microwave source, as seen by looking from an output port of the microwave source into the tuning network. The tuning or matching network is automatically adjusted so substantially all the power from the microwave source goes into the load with little or no power reflected back to the source.
Non-plasma microwave loads driven by such sources and tuning networks are typically microwave antennas of radar systems, satellite systems and communications systems. Such loads are usually fairly well behaved, fairly well characterized and operate in a single microwave mode. However, plasma loads have much more dynamic impedances than the typical non-plasma load, making the plasma loads more difficult to characterize and more difficult to match to a microwave source. In addition, plasmas can and usually support many microwave modes, i.e., descriptions of the spatial microwave power distribution. As the microwave mode changes, due to changes in the plasma, the output of the microwave source is significantly affected.
Conventional microwave matching networks are susceptible to arcing and breakdown, particularly under the high power conditions encountered in driving a plasma load. Typically, a microwave source driving a plasma must produce at least 1 kW and in some instances has a requirement of up to 5 kW, power levels which frequently cause arcing and breakdown in the prior art microwave matching networks.
A typical microwave matching network senses power reflected from the plasma load. In response to the sensed reflected power, the microwave matching network adjusts the stub tuner or sliding short so the matching network reflects the same power as is supplied to it by the plasma, but with a 180.degree. phase change. Thereby, the power reflected by the matching network cancels the power reflected back to the microwave source by the plasma load. This prior art structure inherently establishes substantial microwave standing waves between the microwave matching network and the plasma load. Hence, microwave energy frequently is reflected many times between the matching network and plasma load. Since the matching network is primarily reactive, the only significant loss in the system including the matching network and the plasma load is in the plasma.
In response to the plasma load changing, the load is not matched to the source and the plasma does not absorb a substantial amount of the energy supplied to it, causing the standing waves in the matching network to grow to a large amplitude. The standing waves in the matching network frequently distort the microwave mode applied to the plasma load, leading to unstable performance of the plasma, with deleterious effects on a processed workpiece. The increase in the standing waves greatly increases the risk of arcing in the waveguide connected between the microwave source and the plasma load. If arcing in the waveguide does not occur, the waveguide dissipates a substantial amount of energy, causing the waveguide to heat and absorb energy intended for the plasma load. As a result, the plasma load does not receive substantially all the output power of the microwave source. An operator of the system or an automatic controller has no way of knowing the plasma is receiving a reduced power level, without complicated monitoring equipment for losses in the microwave transmission system between the microwave generator and the load.
Cuomo et al., U.S. Pat. No. 5,179,264 discloses a solid state microwave oscillator for driving a plasma load in an electron cyclotron resonance cavity. Output power of the solid state oscillator is controlled by an optical pyrometer responsive to temperature of a test sample within a plasma processor microwave cavity. The optical pyrometer monitors power radiated by a sample in the chamber to produce a signal proportional to the temperature of the sample. As the temperature of the sample monitored by the optical pyrometer varies, there are proportionate increases and decreases in the optical pyrometer output. These changes in the output of the optical pyrometer control a bias voltage or circuit element in the oscillator to control the oscillator power output.
An apparent problem with the Cuomo et al. optical pyrometer technique for monitoring load conditions is that the plasma which surrounds the sample has a relatively high temperature. Consequently, the background of the environment to which the optical pyrometer is exposed appears to have a tendency to mask the temperature of the sample, which presumably is desirably at the same temperature as the processed workpiece. Consequently, the optical pyrometer would appear to have a very low signal to noise ratio output signal. The low signal to noise ratio output signal of the optical pyrometer would appear to prevent accurate control of the output power of the semiconductor microwave oscillator.
Accordingly, an object of the present invention is to provide a new and improved method of and apparatus for controlling microwave power supplied to a microwave excited vacuum plasma processor for a workpiece.
Another object of the invention is to provide a new and improved relatively simple method of and apparatus for inexpensively and reliably controlling microwave power supplied to a microwave excited plasma processor for a workpiece, wherein the arrangement has highly reproducable results from one workpiece to another.
An additional object of the present invention is to provide a new and improved method of and apparatus for supplying microwave excitation to a vacuum plasma processor that can handle a broad range of plasma impedances and provide control of the power supplied to the processor without having moving parts.
An additional object of the invention is to provide a new and improved method of and apparatus for controlling microwave power supplied to a microwave excited plasma processor, wherein a structure connected between a relatively high power microwave source and a plasma processor is not susceptible to arcing and/or breakdown.
An additional object of the invention is to provide a new and improved method of and apparatus for controlling microwave power for exciting a plasma in a vacuum processor for workpieces, wherein microwave power supplied by the source to the plasma is applied directly to the load without reflecting back and forth a plurality of times in a matching network having variable reactances.
An additional object of the invention is to provide a new and improved method of and apparatus for controlling power supplied by a microwave source to a microwave plasma processor for a load, wherein variations in the load, which have a tendency to affect the microwave operating wave mode and the impedance seen by the source are controlled rapidly, in a simple manner without moving parts.
An additional object of the invention is to provide a new and improved method of and apparatus for controlling microwave power supplied to a microwave vacuum plasma processor for a workpiece wherein control of the power is in response to an output signal having a high signal to noise ratio.